The present invention relates to digital projection of images and, more particularly, to correcting for keystone distortion in such projected images.
Image projection systems are used to project images onto a viewing screen. Examples of projection systems include overhead projectors, traditional slide projectors, and digital projectors. FIG. 1 depicts a digital projector 10 projecting an image onto a viewing screen 12, where the optical axis of the projector 14 is normal to the plane of the viewing screen. These systems are typically used by positioning the projector on a level surface (e.g., tabletop, desk, file cabinet, etc.), and projecting the image up at an angle above the plane of the surface, so that the view of the projected image is unobstructed by the audience. Referring to FIG. 2, when the projector 16 is positioned such that the optical axis 18 of the projection lens is at an angle 20 relative to the normal to the viewing screen 22, geometrical distortion is introduced to the projected image. Let the convention be that a positive angle represents a tilt above the normal to the viewing screen (positive tilt), and a negative angle represents a tilt below the normal to the viewing screen (negative tilt). In the case for positive tilt, the distance from the top of the image to the projection lens 24 is longer than the distance from the bottom of the image to the projection lens 26. The path length discrepancy results in a larger magnification at the top of the image than at the bottom, thus yielding an image that has a larger width at the top of the image than the bottom. The projected image is trapezoidal in shape and is referred to as xe2x80x9ckeystonexe2x80x9d distortion since the distorted image resembles the shape of a keystone (FIG. 3a). In the case of negative tilt where the projected image is lower than the projector, e.g., ceiling mounted projectors, the distorted image becomes an inverted keystone shape, where the width of the top of the image is now smaller than the bottom (FIG. 3b).
There are several methods disclosed in the prior art addressing the correction for keystone distortion in projected images of projection systems. Among these are numerous patents that describe optical keystone correction implementations for fixed projection angles (see U.S. Pat. Nos. 5,283,602, 5,355,188 and 5,706,062). Although some effort has been made towards developing a projector with a variable optical keystone correction (U.S. Pat. No. 5,975,704), optical correction is generally limited to a fixed angle. In addition to the restricted correction angle, the more complicated optical design increases the cost in manufacturing and potentially increases size and weight. An alternative to the optical correction method that is now possible since the availability of digital projection systems is electronic or digital keystone correction.
Digital keystone correction is implemented by performing a compensation operation on the original digital image prior to projection. If the original digital image is distorted such that the amount of distortion is the inverse of the distortion introduced by the projection setup, then the final projected image will be rectilinear. One of the benefits of digital keystone correction is that additional optical elements are not needed since the correction is performed electronically. In addition, the correction applied to the original digital image can be varied according to the tilt angle and direction of tilt (positive and negative keystone correction). Despite the added flexibility with digital keystone correction, there are some limitations. Interpolation artifacts such as loss in sharpness and aliasing, which can manifest themselves as low frequency banding patterns and edge discontinuities, may be introduced as a result of digital keystone correction.
There exist in the prior art several patents that disclose different methods of implementing a digital keystone correction process in hardware (U.S. Pat. Nos. 5,764,311 and 5,795,046); however, these methods do not address the reduction of the interpolation artifacts. Kazutaka, et. al., teach a method of reducing edge discontinuity artifacts after keystone correction in Japanese Patent No. 09-247593; however, this method focuses on correcting for artifacts after they have been introduced by the keystone correction process. Once introduced into the original image information, it is very difficult to eliminate these artifacts without further degrading the image.
It is an object of the present invention to correct for keystoning distortion in digital images projected by a digital projector.
This object is achieved by a method for correcting for keystone distortion and reducing aliasing defects in a digital image for use by a digital image projector, comprising the steps of:
(a) receiving an original digital image;
(b) lowpass filtering the original digital image to provide a lowpass filtered digital image having reduced high frequency spatial components which contribute to aliasing defects introduced by digital keystone correction;
(c) processing the lowpass filtered digital image to provide for digital keystone correction; and
(d) sharpening the keystone corrected digital image.
The present invention provides a method that reduces the occurrence of aliasing defects prior to the digital keystone correction process, while preserving the sharpness of the original digital image.
It is an advantage of the present invention that it reduces aliasing defects while at the same time correcting for keystone distortion. A second advantage is that the loss of sharpness due to the interpolation process is compensated for in the present invention.